1972 240Z all stock with a full engine rebuild 4 years ago.  I'm about to go crazy on this one.  Replaced points/plugs/condenser, have rebuilt carbs/new fuel filter (twice), new plug wires.  Plugs burning perfectly across all cylinders.  Checked wiring from coil to dizzy, recrimped connections.  Cleaned all coil contacts.  Here's what happens.  Some days it will start up perfectly, pull great, I drive it for an hour on back roads.  It's perfect, pulls hard and fast.  Idles great.  The world is good.  Park it for an hour, start it up and it's like the engine has lost its mind.  Won't accelerate, bucks, bounces, and just will not run.  I can barely limp home.  Swear at it, cover it up and attack it the next day.  Double check point gap, timing, etc.  Everything is perfect.  Start it up and it's good.  Nothing wrong.  Drive it, can find nothing wrong.  I'm thinking this is a "hot sink" problem…….until one morning after a cold start it did the same thing.  Just wouldn't go.  Really pissed me off, so I drove it humping and bucking for 20 minutes.  Stopped, pulled cap/rotor  checked gap, put it back together and YES voila…. she pulls perfectly again.  I can't jiggle any wires to make it happen.  And finally today, after the usual great run for an hour, stop, start up, soon complete bucking and bouncing starts, I nurse it towards home.  Within 2 blocks of home she's running perfectly again.  So it happens under both "hot sink" and "cold start" conditions, and now between those extremes.   Any thoughts?  Coil?  Fuel pump?  It's driving me nuts.  Totally intermittent.  Sigh  AW

 Intermittent problems are a major PITA. My first guess would be timing issues, but lets look at fuel first. A few questions please. When did the problem start? Did you tune it before or after the problem? Have you tried pulling the choke on during these episodes? If so, what effect did it have? The chokes can be used to rule out a lean condition. How is the idle during the episode? Does the engine rev. easily & smoothly under no load? A sign of lack of fuel volume.

 On the elec. side, try pulling the plug wires one at a time (insulate yourself)  while idling & see what effect it has. A bad condenser, even if new, can be a nightmare to diagnose. They are cheap, put in another new one just to rule it out. Then I would go at it with a multi-meter & check actual voltage in the secondary system while it's running well(baseline) & again when it craps out & compare.

Whens the last time you checked the fuel tank?

I mean- is it clean ?

Fuel tank was my first thought...

C

Thanks for the responses.  After it acted up the first time, I replaced all the items mentioned above: cap, rotor, points, condenser, wires, plugs, and fuel filter. It does idle perfectly when this happens, accelerates ok, but under load it's a no go.  Didn't try to run with choke.  Coil voltage: into resistor 14v, out of resister 10 volts, main to points 8 volts at idle.  I didn't write these number down when I was looking at dwell/rpm/idle….but I think they're close. My meter has 8 scales on it's face and 4 different switch settings.  Timing at 5 degrees BTDC, dwell @ 41 degrees. This is set per FSM for an engine with pollution paraphernalia.  Deceleration devices plugged, no manifold leaks.  The fuel tank, hmmmm  I don't know if PO dropped the tank and cleaned it.  Let me ask a foolish question, how would one meter fuel flow from the tank?  Or would measuring it at the manifold after the fuel pump do the trick?  I was leaning towards a failing coil, but fuel flow could do it.  Another question, I'm going to pull out the FSM  in a minute, but does anyone know off the top their heads what the fuel line pressure/volume be after the pump?  Thanks  Andy

crud in the lines = intermittent fuel issue, stuff collects, then blows through

intermittent electrical is often a bad ground and/or crusty connections - check 'em all.

I've always read 3.5 lbs of pressure with a mechanical pump.  You should put a clear filter coming out of the tank and check it for debris from the tank.  A Fram G-2 worked best for me and they're cheap.

 

One of mine did the same thing when I first bought it.  I could pull the supply side off the filter on the passenger's side and blow into it and clear it out enough to get home.  Did you keep the filters on the banjo bolts at the carbs?  Is it as bad when the tank is full?

 

Here's what was in mine when I cleaned out the tank, it's not rust, just crud.  I think a kid must have dropped a couple of handfuls of dirt down the fill hole in it's past life.

Edited April 15, 20159 yr by siteunseen

I helped a friend with the same symptoms. Turned out to be debris in the tank moving around and plugging the pickup.

Drain the tank and see what comes out. there is a drain plug bolt in the centre.

Have a few pans ready to catch the gas and a large pail to transfer the gas to after one pan fills up. If you run the fuel down by driving it is less headache.

btw the fact it happens when hot and cold indicates debris.

Ok then, as I'm confident of the electrical (new crimped connectors on resister, coil, dizzy), it's drain fuel tank time. And yes,, it runs perfectly with a full tank topped off, but clearing itself sure looks to point to debris moving around.  BTW, great pic on the scale of the debris field.  Can't get to it today, but will report back when I've tried that.  Thanks for all your suggestions.  Andy

Cliff's neighbour's kid used to dump gravel through the filler door when he wasn't around.

Debris that made it to the filters at the carb inlets.

Edited April 15, 20159 yr by Blue

Cliff's neighbour's kid used to dump gravel through the filler door when he wasn't around.

Used to???  They still do!

 

That hole is big enough for a baseball.